Purpose: :
To investigate the role of Kir4.1 and Kir2.1 channelsin generating the scotopic ERG and to compare the distributionof these channels in adult wildtype (Kir +/+) mice (C57/BL6),and in transgenic mice hemizygous for Kir4.1.

Methods: :
Scotopic ERGs were recorded monocularly from anesthetizedadult control C57/BL6 mice, Kir4.1 hemizygous (+/–) miceand wildtype (WT) littermates. Hemizygous mice were used becauseKir4.1(–/–) mice survive less than a month afterbirth. In some control mice, ERGs also were recorded after intravitrealinjection of barium (1– 2 mM) to block Kir channels. Fromthe weakest intensity up to those for which a–waves appeared,amplitudes measured near the peaks of the b–wave (110ms after the flash) and the negative scotopic threshold response(nSTR; 200 ms) were fit with a previously developed model ofthe scotopic ERG in C57/BL6 mice. The model included 5 components:photoreceptor (PIII), rod bipolar cell (PII), nSTR, positiveSTR (pSTR) and a positive scotopic (pSR) response. After recording,the mice were euthanized and the eyes were prepared for immunolabeling.Retinal cryosections were labeled for Kir4.1 or Kir2.1. To assessKir 4.1 and 2.1 localization in Müller cells specifically,sections were double labeled for glutamine synthetase. Labelingwas visualized by confocal microscopy.

Results: :
A–waves– and maximum b–wave amplitudesof the scotopic ERG were similar in all mice. However, the mostsensitive positive potentials (pSTR and pSR, up to 10% percentof the b–wave) were absent in Kir(+/–) mice. Forthe Kir4.1 (+/–) mice, only three model components, PIII,PII and the nSTR were necessary to fit the data measured at110 ms and 200 ms. As previously demonstrated in other species,the nSTR was eliminated by barium indicating a Müller–cellorigin for the response. In control mice, Kir4.1 was concentratedin Müller–cell end feet and around the blood vessels.In Kir4.1 (+/–) mice, Kir4.1 immunolabeling was weakerthan control in the end feet, and much less obvious around bloodvessels. The Kir2.1 immunoreactivity in Kir4.1 (+/–) micewas similar to that in controls, located predominantly in Müller–celllateral processes, around somas and along stem processes.

Conclusions: :
Our results indicate that Kir4.1 channels in Müllercells play a critical role in generating the sensitive positivecomponents, but not the nSTR of the mouse scotopic ERG. However,Kir2.1 (or other Kir channels) are probably involved in thegeneration of the nSTR.